Wind Turbine Blade Construction
Like many, I had a period where I was interesting in flying. I took lessons and learned how, and that led to a strong desire to build my own airplane. Something like Burt Rutan's "Long Ezy." So, I studied and among other things learned how to hot-wire foam to make near perfect airfoils. Somehow, though, I completely forgot about this technique until recently. As it turns out, the same technique that model aircraft builders and even real aircraft builders use to make their wings and such can be used to make wind turbine blades. Why didn't I think of this before? I suppose because what I really wanted to make was a helical Darrieus wind turbine and I couldn't see how to make the airfoil in the necessary troposkein shape. Well, it turns out to be Ezy! You just cut the airfoil you want out of a straight piece of slightly flexible foam and bend the cut foam into the shape you want before glassing it.
The approach described below for the Darrieus turbine blade can also be used for more conventional blades. If you want a horizontal axis machine just hot wire the blade shapes you want and physically twist the foam to get the necessary varying angle of attack as radius increases, then glass it into place.
So, after you have engineered the blade shape you want, familiarized yourself with the hot wire foam cutting technique by searching Google (hot wire foam cutting) and youtube.com (same terms), and cut out the shape you need --- then you need a "form" with which to impose the necessary curve and twist.
In the case of a helical Darrieus turbine blade a symetrical airfoil is bent into a troposkein shape of appropriate dimensions and at the same time twisted to give it a helical shape. So we develop the troposkein first.
A convenient way to get the correct shape is to hang a chain alongside a board so that the length and depth of curve of the chain is as desired, when everything is as you like it carefully mark several points adjacent to the chain. Then, using a flexible material such as a clear grained board with, say, 3/8" by 3/4" dimensions, bend the board to meet the marks on the board and trace the final curve and cut it out as shown above.
Now we need some twist. In this case I'm targeting a Darrieus turbine with three blades, so 120 degrees of twist per blade is my goal. To get this twist I cut two end boards as shown above with the topmost edge cut at 60 degrees. By placing these boards as shown the total twist over the troposkein edge is 120 degrees.
Now we can put a somewhat flexible board over the frame just constructed as shown.
And that gives us a support upon which to lay our hot wire cut foam airfoil, using the bottom cutoff to provide support.
Once the foam is in place and secure, a layer of glass and epoxy can be laid down. Google can show you how. A nice touch is to first fill in the pores in the foam with a mixture of microbaloons and epoxy. A long strip of cloth running the length of the blade should hold everything in place well enough, then when that has hardened the blade can be removed and both sides finished.
The above image shows one blade mounted. The shaft and hexagonal structures are shown for illustration, but there are many ways to accomplish the same thing. Note, though, that the hexagonal structures are aligned with each other so that top and bottom faces are parallel. The blade is mounted so that one end leads or lags the other by one face on the hexagonal structures, thus accommodating the 120 degrees of blade twist and creating a helix.
The figure above shows all three blades mounted.
It is my guess that just about any symetrical airfoil shape mounted so that the chord axis is perpendicular to the rotational radius will work, but I hate to just guess about these things. If anyone out there has some concrete ideas on this issue, please let me know. Better yet, write up your notions and I'll find some way to put them up here so others can take advantage. With attribution, of course. Only stuff you are willing to share freely with everyone, though! I'm an "open source" kind of guy.
Which reminds me. The information given here is given freely for you to use, or not use, in any way you want with no guarantee of any kind. Use at your own risk!